Make sure that you look into the option "enable cross-database ownership chaining" before installing SP3. If you do not check this alternative during install you will change how ownership/rights between different databases on the same server work. If your applications depend on this they will break.

Also OPENDATASOURCE and OPENROWSET queries (via OLEDB) against remote servers is disabled by default with SP3. Though I do not know if this was the case already before SP3.